Recently, in a thin film transistor used in a liquid crystal display (LCD) and the like, a polysilicone film having a high carrier mobility is used as a channel layer. The polysilicone film in the thin film transistor is produced by forming an amorphous silicone over a glass substrate and annealing the amorphous silicone by irradiating a laser beam to the latter. An apparatus to anneal a substance by irradiating a laser beam to the latter is called “laser annealing apparatus”.
Conventionally, the laser annealing apparatus used in production of the thin film transistor employs, as the light source, the excimer laser which can emit a high power, ultraviolet-region laser beam. FIG. 1 schematically illustrates the construction of a conventional laser annealing apparatus using the excimer laser as the light source.
As shown in FIG. 1, the conventional laser annealing apparatus generally indicated with a reference 200 includes a stage 202 on which a substrate 201 to be annealed is mounted, a laser source 203 which emits a laser beam, a telescope 204 which shapes the laser beam emitted from the laser source 203 into a parallel beam having a predetermined diameter, first and second fly-eye lenses 205 and 206 which split the laser beam coming from the telescope 204 into a plurality of beams and then condense them together into a group of point light sources, and a condenser lens 207 which multiplex the plurality of laser beams having coming from the second fly-eye lens 206 and irradiate them onto a predetermined area on the substrate 201.
In the above conventional laser annealing apparatus 200, a single laser beam is split by the first and second fly-eye lenses 205 and 206 into a plurality of secondary light sources, and a plurality of laser beams from the secondary light sources are irradiated to the predetermined area on the substrate 201. Generally, in case a single laser beam is irradiated as it is to the substrate 201, the intensity distribution will be a Gaussian distribution so that the substrate 201 cannot homogeneously be given the light energy. In the conventional laser annealing apparatus 200, however, the laser beam can be irradiated to the substrate 201 with the intensity thereof being homogeneously distributed by splitting the laser beam by the first and second fly-eye lenses 205 and 206 and then multiplexing the split beams together.
By making the laser annealing of the substrate 201 with the laser beam having the intensity thereof thus homogeneously distributed, the laser energy can be given homogeneously to the surface of the substrate 201 and a polysilicone film having a uniform particle size can be produced.
Note however that the excimer laser used as the light source in the conventional laser annealing apparatus is not easy to handle because the output is not stable. On this account, the light or laser source for use in the laser annealing apparatus should preferably be an ultraviolet-region solid-state or semiconductor laser or the like which can provide a stable laser beam energy and has a long life.
However, the laser beam emitted from such a solid-state or semiconductor laser is highly coherent as compared with a laser beam emitted from the excimer laser. Therefore, in case a solid-state or semiconductor laser is adopted as the laser source 203, the laser beams provided as the plurality of secondary light sources by splitting the single laser beam by the first and second fly-eye lenses 205 and 206 will interfere with each other when they are multiplexed and irradiated to the substrate 201. They will be coherent with each other to result in an interference fringe as shown in FIG. 2. Therefore, even in case the light source in the conventional laser annealing apparatus 200 is replaced with a highly coherent semiconductor or solid-state laser, the laser beam cannot be irradiated to the substrate 201 with the intensity thereof being homogeneously distributed and thus there cannot be produced any polysilicone film in which the particle size is uniform.
Also, in case the solid-state or semiconductor laser or the like is used as the light source in the laser annealing apparatus, only a single light source can hardly provide a sufficient power. On this account, for application of the solid-state or semiconductor laser as the laser source in the laser annealing apparatus, laser beams emitted from a plurality of sources may be multiplexed to produce a laser beam which has a high power and can be irradiated to a wide area. To form a polysilicone film, however, a substrate has to be annealed with a laser beam with a homogeneous intensity distribution in the beam diameter. If not, the crystal particle size will not be uniform and thus a thin film transistor thus produced from the polysilicone film will have no satisfactory characteristic.